1. Field of the Invention
The present invention relates to the field of downhole formation fluid sampling and in particular to the determination of bubble point pressure for a fluid down hole.
2. Background of the Related Art
A primary function of a wireline formation fluid sampling and testing tool, such as the Reservoir Characterization Instrument (RCI™) provided by Baker Atlas, is to collect multiple reservoir fluid samples to evaluate potentially productive reservoir units.
These collected fluid samples are brought to an onshore laboratory facility for PVT (Pressure, Volume, Temperature) and compositional analysis. It is important to obtain high quality samples, which can be used with confidence to predict reservoir behavior. The sample quality depends on the contamination level and the sampling pressure relative to the reservoir bubble point pressure. If the contamination is low (less than 10% by volume), and the sample was taken and maintained above bubble point pressure, then the sample is considered to be very high quality.
If the sample is not collected and subsequently maintained above bubble point pressure, then asphaltenes (heavy components) will fall out of solution just below the fluid's bubble point pressure. Once out of solution, it is unlikely that asphaltenes can be recombined back into the oil, so the fluid properties are permanently altered.
This problem is particularly important for deepwater exploration. If a non-representative sample is used to determine fluid properties, then during production asphaltenes may unexpectedly come out of solution near the wellbore or in the marine riser or the production pipelines. However, when the true formation fluid properties are accurately known based on a high-quality fluid sample, such potential problems can be reduced or eliminated. For an exploration well, for which no samples were recovered, having measured bubble point pressures can be of considerable value in planning the development of the reservoir.
The current generation formation testers are able to measure the bubble point pressure (Pb,) of a formation fluid at the time of sample collection. These testers trap a volume of fluid and gradually expand that volume and monitor how the pressure drops. Initially, the pressure drops linearly with expanding volume. The estimated bubble point pressure is the first pressure at which pressure is no longer linearly related to volume. Because the RCI™ pump provides a linear position digitizer on the pump shaft, the RCI™ operator equipment directly measures the volume change and records the corresponding pressure using a quartz gauge. Because RCI™ performs downhole bubble point pressure measurements in the same way that surface laboratories do (by monitoring P versus V), the downhole bubble point pressure measurements correlates well to laboratory measurements. See, e.g., Kasap, E., Huang, K., Shwe, T. and Georgi, D.: “Formation-Rate-Analysis Technique: Combined Drawdown and Buildup Analysis for Wireline Formation Test Data,” SPEREE, June 1999, Vol 2, No. 3, 273.
The capability to perform the downhole bubble point test was introduced some time ago in Michaels, J., Moody, M., and Shwe, T.: “Advances in Wireline Formation Testing,” paper presented at the 1995 SPWLA Annual Logging Symposium, Paris, France, Jun. 26-29, 1995, but in practice, information from the bubble point pressure test is subjective and not as accurate or repeatable as desired. Thus there is a need for an objective repeatable bubble point pressure test.